When faced with that objection I trained my sales reps to explain to the customer that we guarantee the lights and in the UNLIKELY event a string or two are damaged we come right out and replace them.
After Christmas we do a "Free Christmas Light Removal with Roof Cleaning" special.
Just make sure that they are unplugged if you clean a roof with lights on them. SH is an electrolytic and you could get shocked...
Been there done that, bought the shirt, smoked the doob...................nail was tacked through an electric line when the roof was put on. Add grounding rod "alum ladder" and SH and I danced, danced, danced........................